Single vs Double quote strings in PHP

In PHP, string written in single quotes is displayed 'as it is'. Variables and most of escape sequences are not interpolated.

Example,

$name = 'Farhan';
echo 'Welcome $name';
// Output: 'Welcome $name';

Whereas in double quotes, variables and escape sequences are interpolated.

$name = 'Farhan';
echo "Welcome $name";
// Output: "Welcome Farhan"

Edit #1:
Thanks @cseder for correcting to use interpolated instead of interpreted.

Comments

[Chris Sederqvist]

To be a nitpicker, the word to use is interpolated / interpolation, as in:
You can use interpolation to interpolate (insert) a variable within a string. Interpolation works in double quoted strings and the heredoc syntax.

Everything in PHP gets interpreted...

For completeness, PHP also has the complex / curly bracket syntax for embedding expressions (yielding a return value) in a string.

Any scalar variable, array element or object property with a string representation can be included.
Simply write the expression the same way as it would appear outside the string, and then wrap it in { and }.

Since { can not be escaped, this syntax will only be recognized when the
$ immediately follows the {.

<?php
// Show all errors
error_reporting(E_ALL);

$great = 'fantastic';

// Won't work, outputs: This is { fantastic}
echo "This is { $great}";

// Works, outputs: This is fantastic
echo "This is {$great}";

// Works
echo "This square is {$square->width}00 centimeters broad.";


// Works, quoted keys only work using the curly brace syntax
echo "This works: {$arr['key']}";


// Works
echo "This works: {$arr[4][3]}";

// This is wrong for the same reason as $foo[bar] is wrong  outside a string.
// In other words, it will still work, but only because PHP first looks for a
// constant named foo; an error of level E_NOTICE (undefined constant) will be
// thrown.
echo "This is wrong: {$arr[foo][3]}";

// Works. When using multi-dimensional arrays, always use braces around arrays
// when inside of strings
echo "This works: {$arr['foo'][3]}";

// Works.
echo "This works: " . $arr['foo'][3];

echo "This works too: {$obj->values[3]->name}";

echo "This is the value of the var named $name: {${$name}}";

echo "This is the value of the var named by the return value of getName(): {${getName()}}";

echo "This is the value of the var named by the return value of \$object->getName(): {${$object->getName()}}";

// Won't work, outputs: This is the return value of getName(): {getName()}
echo "This is the return value of getName(): {getName()}";
?>

[Farhan Halai]

Thanks @cseder for correcting me out.
And adding some valuable examples too ✌